1. Field of the Invention
The present invention relates to image quality maintenance in an image forming apparatus achieving image forming processing using a two-component developing agent.
2. Description of the Related Art
As a technology for obtaining a color image with high image quality at a high speed, there has hitherto been known a configuration in which in an image forming apparatus of a so-called “quadruple tandem system”, toner images of plural colors are superimposed and transferred on an intermediate transfer belt by process units disposed along the intermediate transfer belt and then transferred at once onto paper or the like.
According to the foregoing related-art technology, the “superimposition and transfer” which is liable to become unstable from the process standpoint is carried out on a stable intermediate transfer belt, thereby achieving the transfer with high image quality as it stands, and thereafter, secondary transfer is achieved at once on a final transfer material such as paper. Thus, multiplicity of use of paper can be improved while controlling the degradation of image quality to the minimum.
In image forming apparatus of such a configuration, a two-component development system which is advantageous for realizing high image quality is frequently employed. In recent years, by aiming to realize higher image quality of this two-component development, the particle size of a carrier to be used is becoming small.
In order to hold a color balance which is particularly important in superimposing colors, such an apparatus for high image quality is provided with a so-called image quality maintenance control mechanism in which in a state other than the time of image printing operation, after transferring a patch image on an intermediate transfer belt, a patch density, a reflectance, or the like is detected by a reflectance sensor or the like provided within the apparatus, thereby adjusting an image forming condition by that value.
As the image forming condition to be changed by the image quality maintenance control mechanism, for example, various conditions such as process conditions including charging bias voltage, development bias voltage, exposure amount and toner concentration (T/C) in a development unit and a combination of tone characteristics by changing an image processing pattern are known, and a combination of plural controls is employed.
However, among these conditions, for example, when a background contrast potential (a difference between charging potential and development potential of photoconductor) or a toner concentration within the development unit is controlled in a large range, in particular, in the case where the development system is a two-component system, there is involved a problem that a carrier particle easily attaches to the photoconductor. This means that in aiming to realize high image quality, the smaller the particle size of the carrier, the narrower the margin within which the condition can be changed.
Furthermore, in addition to the realization of high image quality, in order to make it compatible with realization of low costs or long life of the apparatus, when a brush charging unit which is strong against staining and low in costs is used as a charging member of the photoconductor, charging unevenness inherent to the brush is caused. In particular, streak-like potential unevenness in a direction of charging the photoconductor higher than a desired charging potential is inherent to a brush-like member and has a harm to easily generate the attachment of a carrier to the photoconductor in a development section.
Furthermore, for example, when a carrier attaches to a side of the photoconductor in a development section of an image forming station in the most upstream side of a quadruple tandem apparatus, this carrier is sandwiched at a transfer position against an intermediate transfer belt, whereby a surface of the photoconductor is rubbed and scratched. In addition, in the case where the carrier is transferred at the transfer position to a side of the intermediate transfer belt, since the carrier which has attached to the photoconductor in a first image forming station reaches the transfer section of second, third and fourth image forming stations, in particular, damages against the photoconductor become extreme in a later station. Then, the surface of the photoconductor is shaven by the carrier, and a number of crater-like recesses are generated. Thus, the image resolution is lowered, and a toner or an external additive of the toner further adheres to the surface of the photoconductor from the recesses, whereby faults such as a streak and a white spot are generated in an image. In addition, since a phenomenon in which the carrier attaches to the side of the photoconductor continues over a long period of time, the amount of the carrier within the development unit is reduced. Accordingly, the amount of a developing agent within the development unit is reduced, and density unevenness or the like is liable to be generated in printing a solid image.
That is, in order to aim to realize high image quality, in the case of a color printing apparatus using an intermediate transfer belt of a quadruple tandem system which employs two-component development with a carrier of a small particle size, it cannot be freely achieved in view of a problem of a harm of the carrier attachment phenomenon to largely control a background contrast potential or a toner concentration within a development unit for the purpose of aiming to improve the precision of a color balance or the like which is essential for realizing high image quality; and in the case where characteristics of a material vary with a change of the circumferential environment or a change with time, a process condition cannot be sufficiently controlled. Thus, it was impossible to obtain a synthetically sufficient high image quality. Furthermore, even when a brush-like charging member is employed for the purpose of aiming to realize both low costs and a long life at the same time, the same problems were caused.